The present invention relates to a finishing screed for compacting and smoothing an asphalt mix and which is useful in forming roadway surfaces.
Present asphalt paving machines commonly comprise a self propelled tractor which deposits the asphalt on the ground behind the tractor, and which then distributes the deposited mix transversely by means of a pair of rotating augers which are transversely directed. The tractor mounts a trailing finishing screed by means of a pair of rearwardly extending tow arms which are pivoted to the opposite sides of the tractor, and so that the forward ends of the tow arms may be raised and lowered. The finishing screed is attached between the rear ends of the tow arms so as to follow the tractor in a free floating arrangement. The screed serves to spread and compact the asphalt which has been deposited by the tractor unit. Known screeds typically comprise a strike off bar which is followed by a tamper bar, with the tamper bar being mounted on an eccentric shaft so as to vertically reciprocate upon rotation of the shaft. The tamper bar is in turn followed by a horizontal screed plate, which is usually heated.
A major problem associated with known screeds of the described type is the fact that the tamper bar stroke cannot be adjusted to accommodate different mat thicknesses. For example, asphalt mix typically compacts about 1/4 inches per inch of thickness at full compaction, and thus the initial uncompacted thickness should be about 11/4 inches to achieve a 1 inch finished mat. For a 2 inch finished mat, an initial thickness of about 21/2 inches is required, and so forth.
The stroke of the tamper bar in most conventional screeds is set to be about 3/16 inch, and longer strokes have not been practical since the bearings are not able to support the forces that would be generated by the rapidly reciprocating bar moving a greater distance. This eccentric stroke determines the maximum compaction that can be obtained, and thus known screeds are not able to fully compact mats having a thickness greater than about 1 inch. To remedy this deficiency, it is usual to provide a number of self propelled rollers which follow the paving machine, and which serve to further compact the asphalt mat. In particular, a breakdown roller, a rubber tired roller, and a sealing or finishing roller are usually provided for this purpose.
It is accordingly an object of the present invention to provide a finishing screed of the described type and which is able to achieve substantially full compaction of an asphalt mat over the full range of usual mat thicknesses, including very thick mats.
It is another object of the present invention to provide a finishing screed of the described type which minimizes the need for the use of self propelled rollers to complete the compaction operation.
It is still another object of the present invention to provide a finishing screed of the described type and which is able to meter a readily selectable thickness of the asphalt material to the compactor of the screed so as to achieve full compaction of the desired final mat thickness.